Apparatus and methods for remakeable connections to optical waveguides

ABSTRACT

A single-mode optical waveguide with a core, surrounded by a cladding consisting of an inner soft layer and an outer harder layer is described. The outer layer has a grating structure on its inner surface, whose spatial frequency is the same as that of the guided mode. The thickness of the inner cladding is sufficient to keep the grating outside the mode field in undeformed regions of the waveguide, so that normally no out-coupling of the light results. Connections are made by crossing two such waveguides at an angle and pressing them together. This results in deformation of the two waveguides such that the gratings are brought into proximity with the cores. Light is coupled out of one waveguide and into the other in the deformed region, resulting in a self-aligning optical connection. The out-coupled light propagates normal to the waveguide axis, so errors in the crossing angle cause little change in efficiency. Because the cladding system is sufficiently resilient to recover after deformation, the connection is remakeable.

FIELD OF THE INVENTION

[0001] This invention relates to optical information transmission. Morespecifically it relates to apparatus and methods for providingremakeable connections for single-mode optical waveguides.

BACKGROUND OF THE INVENTION

[0002] Metal transmission lines are an increasingly serious bottleneckto the operation of computers, switch-routers, and other informationhandling systems. The speed of processors and logic has been increasingmuch faster than the available wiring bandwidth, resulting inunfavorable tradeoffs of power consumption, complexity, and transmissionspeed. Hence there is increasing interest in using opticalinterconnection between boards in computers, and even between chips on aboard or a multi-chip module (MCM).

[0003] Guided-wave optics replaces electrical conduction in wires, withlight propagating in dielectric optical waveguides. A dielectric guideis typically a cylindrical structure (often, not necessarily, circular),made up of annular layers. The innermost cylinder is the core, and issurrounded by one or more cladding layers, having refractive indicesthat are lower than that of the core. Within certain ranges offrequency, position, and angle, light traveling axially in such astructure is guided, that is, it shows no tendency to spread outradially with distance as ordinary light beams do. There is always atleast one guided mode in any structure where the core index exceeds thecladding index (two, if polarization is taken into account). A waveguidein which only one such mode can propagate at a given frequency is calleda single-mode waveguide. Depending on the waveguide dimensions andrefractive indices, there may be other modes as well, in which case theguide is said to be a multimode waveguide.

[0004] The optical mode field falls off exponentially with radialdistance in the cladding region, and its decay constant depends on thedifference between the core and cladding refractive indices. In order toallow the core to be relatively large (several wavelengths in diameter)and reduce losses due to roughness and index non-uniformity at thecore-cladding boundary, the core and cladding indices are usually chosento be close to each other (1%-5% difference), which makes the decayconstant in the cladding region a very sensitive function of the indexdifference. These guided-wave optical devices have many applications, ofwhich the most important is long-haul fiber optic communication.

[0005] Modulated optical beams travel almost exactly the same way asunmodulated ones, regardless of how fast the modulation is; there is notendency for higher modulation frequencies to be lost, in sharp contrastwith the high frequencies in a metal wired connection. Guided waveoptics is thus a natural candidate for such connections, but the set ofrequirements inside a computer are quite different from those that arefamiliar from long-haul communications. The hardware required isdifferent as well. Inside the computer, many parallel connections areneeded, each with a single unidirectional data stream coming from onemodulated source per line to one detector per line. There are alaoimportant applications for one-to-many connections, where one sourcedrives many inputs. These sources and detectors must be inexpensive,because there are so many required (hundreds for a typical serverprocessor board). On the other hand, the very low loss of fibers(0.15-0.5 dB/km) is unnecessary, and no wavelength-division multiplexingis required. Thus, optical interconnections inside computers are likelyto rely on polymer waveguides arranged in ribbons, and mass-producedespecially for the purpose.

[0006] Furthermore, these parallel connections must be remakeable, sothat the board can be unplugged for service and a spare plugged in,without requiring that the optical links be replaced. Multimodewaveguides and fibers are currently the most common, since theircomparatively large cores allow them to be spliced and coupled withloose mechanical tolerances compared to those of ordinary single-modefibers, as used in telecommunications. Loose tolerances translate intolow cost and usually into mechanical robustness as well.

[0007] Single-mode waveguides have significant advantages over multimodewaveguides, especially in their small diameter and the resulting highinterconnection density. Since light in a single-mode waveguide has awell defined guide wavelength, single-mode waveguides allow the use oflong gratings and directional coupler structures that rely on awell-defined guide wavelength and field distribution. Hence aremakeable, single-mode, multi-waveguide connector that allowed loosemechanical tolerances would be a significant advance in opticalinterconnection technology.

SUMMARY OF THE INVENTION

[0008] The present invention provides an apparatus and method for makingmultiple remakeable single-mode waveguide connections. When twosuitably-designed waveguides are crossed at an angle there will be astress concentration in the region where the cores cross. If someportions of the waveguides are soft, they will deform the most at theplace where the cores of the waveguides cross. The present inventionrelies on the physical deformation of soft parts of the cladding in thestress concentration zone to cause coupling to occur between the cores,without unacceptable loss elsewhere in the waveguide.

[0009] In accordance with a first aspect of the invention, the couplingis produced by physical proximity of the cores of the deformedwaveguides, which causes their mode fields to overlap, resulting indirectional coupling similar to that in a fused-fiber coupler.

[0010] In accordance with a second aspect of the invention, the couplingis produced by a grating structure in each waveguide, formed in a hardercladding region surrounding an inner soft cladding, the combined actionof which is to couple light out of one waveguide and into the other.

[0011] In accordance with a third aspect of the invention, a gratingstructure in the hard cladding region couples light from a laser sourceinto the waveguide through the side of the waveguide.

[0012] In accordance with a fourth aspect of the invention, a gratingstructure in the hard cladding region couples light from the guided modeout into one or more photodiodes through the side of the waveguide.

[0013] In accordance with a fifth aspect of the invention, a prismstructure in the hard cladding region couples light from the guided modeout into a photo diode through the side of the waveguide.

[0014] In accordance with a sixth aspect of the invention, a remakeablewaveguide connection is made by forming several waveguides into aribbon, with spaces between them, crossing the ends of two such ribbonsat a small angle and pressing them together so as to deform the softcladding and initiate the coupling.

[0015] In accordance with a seventh aspect of the invention, the changein optical properties in the stressed region is made by a piezo-opticalshift in the refractive index of a portion of the cladding, in additionto physical deformation, so that the decay length of the mode field inthe cladding is increased sufficiently for interaction to take placebetween the mode field and a coupling structure, such as a secondwaveguide, a higher-index region, or a grating.

BRIEF DESCRIPTION OF THE DRAWINGS

[0016] These and other aspects, features, and advantages of the presentinvention will become apparent upon further consideration of thefollowing detailed description of the invention when read in conjunctionwith the drawing figures, in which:

[0017]FIG. 1A is an enlarged cross-sectional view of an opticalwaveguide in accordance with the invention.

[0018]FIG. 1B is a cross-cross-sectional view of an optical waveguide inaccordance with a first embodiment of the invention taken along line1B-1B of FIG. 1A.

[0019]FIG. 2A and FIG. 2B are similar views of the same opticalwaveguide as in FIG. 1A and FIG. 1B when reversible indented by a secondmatching waveguide.

[0020]FIG. 3 is a cross-sectional view of two waveguides in accordancewith a second embodiment of the invention pressed together.

[0021]FIG. 4A is an enlarged cross-sectional view of an opticalwaveguide in accordance with a third embodiment of the invention.

[0022]FIG. 4B is a cross-cross-sectional view of an optical waveguide inaccordance with the invention taken along line 4B-4B of FIG. 4A.

[0023]FIG. 5 is a plan view of a remakeable waveguide ribbon connectorin accordance with the invention.

[0024]FIG. 6 is a plan view of a remakeable parallel optical link inaccordance with the invention.

[0025]FIG. 7 is a cross-sectional view of two waveguides in accordancewith a fourth embodiment of the invention, pressed together.

DESCRIPTION OF THE INVENTION

[0026] In accordance with the invention, an as more fully describedbelow each waveguide is preferably a single-mode waveguide and comprisesa core made from a hard polymer such as PMMA, polycarbonate, fluorinatedpolymers, benzocyclobutanes, epoxies such as SU-8, or cycloolefiniccopolymers e.g. polynarbonene, or fluorinated polyimides. The core issurrounded by a cladding consisting of an inner soft layer made of anelastomer such as a silicone elastomer or silicone copolymer elastomers,and thin outer layer made from a higher modulus polymer sufficientlytough to recover from repeated deformation, e.g. polyimide,polynarbonene, cross-linked silicone, or fluorinated polymer such asfluorinated acrylic.

[0027] It is desirable that the hard outer cladding have a higherrefractive index than the soft cladding, to allow a grating structure tobe blazed appropriately to achieve directional selectivity. The hardouter cladding does not need to be of the same optical quality as thecore or inner cladding, because it is illuminated only in theout-coupling region, which significantly broadens the possible choicesof material.

[0028]FIG. 1A and FIG. 1B show a polymer optical waveguide 10 comprisingcore region 12, surrounded by a soft cladding region 14 made from acompliant polymer, surrounded in turn by a hard cladding 16 made from aflexible polymer of higher modulus, as described above. A portion of theinterface between the two cladding layers contains a grating structure18, preferably on its inner surface. The spatial frequency of thegrating 18 is selected to be the same as that of the guided mode. Thethickness of the inner cladding is sufficient to keep the gratingoutside the mode field in undeformed regions of the waveguide, so thatnormally no out-coupling of the light results, and the waveguidepropagation loss is comparable to that of other polymer waveguides,about 0.02-0.1 dB/cm.

[0029]FIG. 2A and FIG. 2B show the same polymer optical waveguide afterbeing reversibly indented by a second matching waveguide (not shown inthese figures), crossing it at an angle. In the deformed region, thegrating structure 18 has been forced closer to the core 12, leading togreatly enhanced out-coupling of the light in the waveguide, as shown at19. As shown, the grating period is equal to the guide wavelength of thelight, so that the peak of the angular spectrum of out-coupled light isnominally perpendicular to the axis of the waveguide.

[0030]FIG. 3 shows polymer waveguides 20 each comprising a core region22, surrounded by an annular soft cladding 24 and a hard cladding layer26, in a manner similar to

[0031]FIG. 1A and FIG. 1B, but without a grating at the interface. Twowaveguides are shown pressed together, with the result that the modefields of their cores overlap, and directional coupling occurs betweenthem.

[0032]FIG. 4A and FIG. 4B show a polymer waveguide 30 consisting of acore region 32, surrounded by an annular soft cladding layer 34 and ahard cladding layer 36 in a manner similar to FIG. 1A and FIG. 1B, butwith a prism array 38 at the interface, instead of a grating. Thisallows the use of multimode waveguides, but requires that the materialof the prism array 38 have a refractive index higher than that of thesoft cladding 34. In FIG. 4A and FIG. 4B it is shown in a configurationsuitable for a prism index greater than either of the soft or hardcladding materials.

[0033]FIG. 5 shows a remakeable waveguide ribbon connection designed forconnecting two ribbons 42 and 44 each comprising several parallelsoft-cladding waveguides 40 spaced apart slightly, and crossed at aslight angle, in the range of 0.5 to 2.0 degrees, depending on thestrength ∘ the coupling per unit length. The two waveguides are pressedtogether by a clamp 46, to from an interaction zone 48, and due to theself-aligning feature of the coupling, the alignment tolerance is equalto the full separation between waveguides. In a preferred embodiment ofthe invention, the waveguides in each array of waveguides are spacedfrom one another by a few times the diameter of the waveguides, andformed into a ribbon. The two ribbons are held in mechanical guides 47to keep them in position and to relieve stress. It is desirable thatthere be some surface corrugations (as in ordinary wire ribbon cables),generally running parallel to the cores, along the length of thecoupling surfaces of the waveguides, to allow the soft claddingsomewhere to flow into as it deforms under stress.

[0034] Thus, mass connections are made by crossing two such ribbons at aslight angle, near their ends, and pressing them together with a clamp.The angle and interaction length are preferably chosen so that eachindividual waveguide crosses exactly one waveguide in the other ribbon.Because of the surface relief in the vicinity of the cores, the pressureresults in compressing of the two waveguides, preferentially in theregion where the two cores cross one another. This compressing bringsthe gratings into proximity with the cores in the crossing region, andcoupling light out of the core of the illuminated waveguide. Because thegrating period is the same as the guide wavelength, the peak of theangular spectrum of the out-coupled light travels substantially normalto the plane of the ribbon, and since the waveguides are compressed totheir thinnest in the direction joining the two cores, the out-couplingis greatest where the cores overlap. Thus the in-coupling grating phasematches the out-coupled light into the grating of the receivingwaveguide, and the result is a self-aligning optical connection. Theout-coupled light propagates normal to the waveguide axis. Thus, errorsin the crossing angle cause little change in efficiency. The connectionis remakeable, because the cladding system is sufficiently resilient torecover after deformation.

[0035]FIG. 6 shows one end of a remakeable parallel optical link 50based on these waveguides. Here a region near the end of each ofcompliant waveguides 60 of a waveguide ribbon 61 is clamped against alauncher 70, by a clamp 76. The launcher 70 has waveguides 72 withsimilar properties to the compliant waveguides 60, but having one endwhere the soft cladding is replaced by a harder one 78, so that thelocation of the waveguide cores is accurately known. Thus, the modulusof the cladding is a function of position along the waveguides. Thehardened ends are used to end-fire couple to an array 79 of VerticalCavity Surface Emitting Lasers (VCSELs) at one end of the link, asshown, and an array of photo diodes at the other (not shown). VCSELs arepreferred sources for optical links because they are available inmonolithic arrays with lithographically defined spacings, but otherstructures known in the art, e.g. free space grating couplers orindivisually mounted lasers or detectors are also suitable.

[0036]FIG. 7 shows a pair of guides 80, having cores 82, and using aninner cladding 84 made from a polymer with a high piezo-optical (orstress-optical) coefficient, such as a polybutadiene, apoly(styrene-b-butadiene, a Silicone, or a polyisoprene; and a hardouter cladding 86. In the compressed region, the refractive indices ofthe inner cladding 84 increase significantly, because stressed materialis usually optically anisotropic (more than one index is needed todescribe it). This piezo-optic shift makes the core and inner claddingindices much closer in value, so that the decay constant is reduced andthe 1/e contour of the mode field 88 extends far enough into thecladding to interact with the other waveguide, and result in theefficient coupling of light from one waveguide to the other. There is aflattening of the waveguides 80 along their circumferences, wherecontact is made at 90. A small amount of coupling gel may be used toassure good optical coupling between waveguides 80. The stressed regionis approximately defined by dotted line 92.

[0037] The piezo-optical shift should be a significant fraction of thecore-cladding refractive index difference in order for the mode-fielddiameter in the coupling region to be increased sufficiently forefficient coupling to occur. Thus, the piezo-optical cladding shouldhave a high enough piezo-optical coefficient so that its index shiftssufficiently at a pressure small enough not to cause permanentdeformation. Typically the piezo-optical shift is in the order of onlyone to two percent.

[0038] The piezo-optical effect can also be used to couple the core to agrating structure in the cladding, in a manner similar to that describedabove.

[0039] Although the description of the invention set forth above hasreferred to light and to optical elements, it will be understood bythose skilled in the art that the invention is not limited to operatingwith visible light. With dimensions suitably chosen for the wavelengthand refractive indices, the present invention can be used at anywavelength at which sufficiently transparent materials having suitablemechanical properties exist. This range extends at least from extremelyhigh radio frequencies through the near ultraviolet.

[0040] It is noted that the foregoing has outlined some of the morepertinent objects and embodiments of the present invention. The conceptsof this invention may be used for many applications. Thus, although thedescription is made for particular arrangements and methods, the intentand concept of the invention is suitable and applicable to otherarrangements and applications. It will be clear to those skilled in theart that other modifications to the disclosed embodiments can beeffected without departing from the spirit and scope of the invention.The described embodiments ought to be construed to be merelyillustrative of some of the more prominent features and applications ofthe invention. Other beneficial results can be realized by applying thedisclosed invention in a different manner or modifying the invention inways known to those familiar with the art. Thus, it should be understoodthat the embodiments has been provided as an example and not as alimitation. The scope of the invention is defined by the appendedclaims.

Having thus described our invention, what we claim as new and desire tosecure by Letters Patent is as follows:
 1. A deformable opticalwaveguide, comprising: a core; and a deformable cladding layersurrounding the core, for allowing thickness of the cladding to bealtered by applying stress to the waveguide.
 2. The waveguide as inclaim 1, further comprising: a resilient cladding layer surrounding thedeformable cladding layer.
 3. The waveguide as in claim 1, in which thedeformable cladding layer comprises an elastomer.
 4. The waveguide as inclaim 2, further comprising: a grating structure at a portion of aninterface between the deformable cladding layer and the resilientcladding layer.
 5. The waveguide as in claim 2, further comprising: agrating structure at a portion of an outside surface of the resilientcladding.
 6. The waveguide as in claim 2, further comprising: a gratingstructure in an interior portion of the resilient cladding.
 7. Thewaveguide as in claim 2, further comprising: a grating structure in aninterior portion of the deformable cladding.
 8. The waveguide as inclaim 1, further comprising: a prism structure, for coupling light froma guided mode into unguided propagation.
 9. The waveguide as in claim 1,wherein the deformable cladding is mechanically anisotropic.
 10. Thewaveguide as in claim 1, wherein the deformable cladding layer has amechanical modulus which is a function of position along the waveguide.11. The waveguide as in claim 1, wherein the deformable cladding layercomprises a material having a sufficiently large positive piezo-opticalcoefficient to allow outcoupling of light from said waveguide, when saidwaveguide is stressed.
 12. The waveguide as in claim 11, wherein thechange in piezo-optical coefficient due to stress is in the range of oneto two percent.
 13. The waveguide as in claim 11, in which thepiezo-optical material is a polymer.
 14. The waveguide as in claim 13,in which the polymer is selected from the group of a polybutadiene, apoly(styrene-b-butadiene, a silicone, and a polyisoprene.
 15. Thewaveguide, as in claim 13, further comprising: a coupling structureadjacent a portion of the deformable cladding, the coupling structurebeing insignificantly illuminated by a mode field of the waveguide whenthe portion of the deformable cladding is unstressed, said couplingstructure being significantly illuminated when the portion of thedeformable cladding is stressed.
 16. The waveguide as in claim 15, inwhich the coupling structure is a grating.
 17. A waveguide, as in claim15, in which the coupling structure comprises a prism.
 18. A waveguidewith resilient and deformable regions, comprising: a core; a claddingregion, surrounding the core, a portion of which is deformable and aportion of which is resilient, the two portions being at differentpositions along the waveguide.
 19. A waveguide ribbon, comprising: aplurality of waveguides arranged in an array, each waveguide comprisinga core; a deformable cladding layer surrounding the core, for allowingthickness of the cladding to be altered by applying stress to thewaveguide; and a flexible planar carrier for the waveguides, for holdingthe waveguides in position.
 20. The waveguide ribbon of claim 19,wherein the array is a linear array.
 21. The waveguide ribbon of claim19, in which the coupling surface has corrugations running parallel tothe cores, allowing space for the deformable cladding to flow into whenlocal pressure is applied.
 22. A remakeable optical waveguide connector,for connecting pressure sensitive waveguides, the connector comprising:first and second mechanical guides, for holding pressure sensitivewaveguides at an angle; and a removable clamp, for pressing thewaveguides together, the pressure being held within a predeterminedrange, the pressure being great enough to cause coupling between thewaveguides and small enough to prevent damage to the waveguides.
 23. Theremakeable optical waveguide connector as in claim 22, in which themechanical guides are arranged so as to hold the pressure-sensitivewaveguides at an angle to each other.
 24. The remakeable opticalwaveguide connector as in claim 23, in which the angle is in the rangeof 0.5 degrees to 2 degrees.
 25. The remakeable optical waveguideconnector as in claim 22, in combination with a plurality of opticalwaveguides, each optical waveguide comprising a core; and a deformablecladding layer surrounding the core, for allowing thickness of thecladding to be altered by applying stress to the waveguide.
 26. Aremakeable optical waveguide launcher, for coupling to an array ofwaveguides, the launcher comprising: an array of light sources; acoupling structure having an array of light guides for receiving lightfrom said light sources, a mechanical guide, for holding the waveguidesat an angle to said coupling structure; a removable clamp, for pressingthe waveguides and said coupling structure together, the pressure beingheld within a predetermined range, the pressure being great enough tocause coupling between the waveguides and the coupling structure andsmall enough to prevent damage to the waveguides.
 27. The remakeableoptical waveguide launcher as in claim 26, wherein each opticalwaveguide comprising a core; and a deformable cladding layer surroundingthe core, for allowing thickness of the cladding to be altered byapplying said pressure to the waveguide.
 28. A method for coupling lightbetween an optical waveguide and unguided propagation, comprising:providing a waveguide having a core; and a deformable cladding layersurrounding the core, for allowing thickness of the cladding to bealtered by applying stress to the waveguide; and compressing a portionof the waveguide so as to decrease a refractive index of the cladding toa value not less than that of the core.
 29. A method for coupling lightbetween optical waveguides, comprising: providing first and secondwaveguides, each waveguide having a core; and a deformable claddinglayer surrounding the core, for allowing thickness of the cladding to bealtered by applying stress to the waveguide; placing the waveguidesagainst one another so that they cross one another at an angle; pressingthe waveguides together so that the deformable cladding regions arethinned in a region where the cores cross, causing a mode of the firstwaveguide to be coupled to a mode of the second waveguide.
 30. Themethod as in claim 29, further comprising launching light into said modeof the first waveguide.
 31. The method as in claim 29, in which eachwaveguide has a grating structure in the cladding.
 32. A method forcoupling a waveguide mode with unguided propagation, comprising:providing a waveguide, the waveguide having a core; and a deformablecladding layer surrounding the core, for allowing thickness of thecladding to be altered by applying stress to the waveguide; providing anoptical surface; pressing a portion of said waveguide against saidoptical surface so that the deformable cladding layer is thinned in aregion of highest pressure, causing a mode of the waveguide to becoupled into free propagation.
 33. The method as in claim 32, in whicheach waveguide has a grating structure in the cladding.
 34. A method forcoupling light between optical waveguide ribbons, comprising: providingfirst and second ribbons, each optical ribbon including a plurality ofwaveguides arranged in an array, each waveguide comprising a core; adeformable cladding layer surrounding the core, for allowing thicknessof the cladding to be altered by applying stress to the waveguide; and aflexible planar carrier for the waveguides, for holding the waveguidesin position; placing the ribbons against one another so that they crossone another at an angle; pressing the ribbons together so that thedeformable claddings are thinned in regions where the cores cross,causing a mode of each waveguide in the first ribbon to be coupled to amode of a waveguide in the second ribbon.
 35. The method as in claim 34,further comprising launching light into said mode of a waveguide in thefirst ribbon.
 36. The method as in claim 34, in which each waveguide hasa grating structure in the cladding.